For an analysis of a two-dimensional image, the image is scanned in a raster-like pattern by means of a light, laser or electron beam. This process results in electrical signals produced in a receiver which may be a photocell, a photoelectric multiplier or a television receiver tube, for example. These scanning signals are either supplied for evaluation to a discriminator delivering binary signals so that a binary image (black-and-white image) is generated for further image processing, or they are subdivided into gray stages, for example 256, resulting in a gray image which, in the majority of cases, has a significantly higher information content than a binary image and the further processing of which is consequently substantially more complex.
A good dissection of an image is achieved using a generator for producing a high-frequency voltage and synchronizing the generator with the movement of the scanning beam, for example, as disclosed in U.S. Pat. No. 2,494,441 and German published and examined patent application DE-AS No. 1,423,636. This generator supplies a voltage that subdivides the image or the lines into individual raster or image elements.
Numerous methods and circuit arrangements for the further processing of binary images subdivided into individual image elements which can be described by the term "texture analysis" are disclosed in German Pat. Nos. 2,128,690 and 2,354,769, for example.
In view of the numerous gray stages of each individual image element, the processing of digitalized gray images subdivided into image elements necessitates methods and circuit arrangements other than those required for the processing of binary images. As in binary images, in many cases a so-called evaluation window covering several image elements is moved across the image, and the values lying within the boundaries of this window are processed in a defined manner and allocated to the central image point of the evaluation window. Of particular interest in this connection are methods operating "video-rapidly", that is, methods in which the number of processed images issued per unit of time equals the number of images entered on the input side. This means that, for example, twenty-five (25) full images including 625.times.833 image elements have to be processed per second.
Published European patent application No. 0 068 358 A2 discloses an arrangement wherein a 4.times.4 window is evaluated video-rapidly by means of parallel processing, with the digitalized gray value of each image element being multiplied by a weighting factor (differing for each location in the evaluation window) followed by the summation of all image elements thus weighted.
Published European patent application No. 0 069 542 A2 describes an arrangement wherein a 3.times.3 window is evaluated video-rapidly by multiplying the 4-bit gray value of each image element by a weighting factor, followed by the summation of all image elements thus weighted and a comparison of the sum with a limit value.
In addition to such sums of the weighted image elements of an evaluation window, the minimum, maximum and particularly the so-called median of all values within the evaluation window are of special significance for quantitative image analysis. These values may be referred to as operators reflecting an order of rank. The characteristic of the median is to maintain sharp edges in a sharp condition; for an uneven number of m values, it is defined as the value for which (m-1)/2 values are smaller or equal and (m-1)/2 values are greater or equal. Within the order scheme, it is thus the mean value of a group of values.
It is obvious that the determination of the median requires a great many operations because all values first have to be arranged in their order of rank. While programs suitable for this purpose have already been used in data processing installations, they all have the disadvantage that the numerous operations to be executed require a computing time which is too long for video-rapid evaluation, unless, of course, sophisticated computer systems are used which are too expensive for standard apparatus.